$(document).ready(function(){
	//imgタグに[class="rollover"]で[src="hogehoge_on"]を参照
	$("img.rollover").each(function() {
		var off = this.src;
		var dot = this.src.lastIndexOf('.');
		var on = this.src.substr(0, dot) +
			'_on' + this.src.substr(dot, 4);
		$(this).hover(
			function() { this.src = on; },
			function() { this.src = off; });
	});
	
});